Spinal fixation systems may be used in orthopedic surgery to align, stabilize and/or fix a desired relationship between adjacent vertebral bodies. Such systems typically include a spinal fixation element, such as a relatively rigid fixation rod or plate, that is coupled to adjacent vertebrae by attaching the element to various anchoring devices, such as hooks, bolts, wires or screws. The spinal fixation element can have a predetermined contour that has been designed according to the properties of the target implantation site and, once installed, the spinal fixation element holds the vertebrae in a desired spatial relationship, either until desired healing or spinal fusion has occurred, or for some longer period of time.
Spinal fixation elements can be anchored to specific portions of the vertebra. Since each vertebra varies in shape and size, a variety of anchoring devices have been developed to facilitate engagement of a particular portion of the bone. Pedicle screw assemblies, for example, have a shape and size that is configured to engage pedicle bone. Such screws typically include a threaded shank that is adapted to be threaded into a vertebra, and a head portion having a spinal fixation element receiving element, which, in spinal rod applications, is usually in the form of a U-shaped slit formed in the head for receiving the rod. In many pedicle screws, the head is movable and preferably pivotable in all directions, relative to the shaft. The ability to move the head relative to the anchoring portion of the screw facilitates alignment and seating of a rod connecting a plurality of screws
A set-screw, plug, cap or similar type of closure mechanism is used to lock the rod into the rod-receiving portion of the pedicle screw. In use, the shank portion of each screw is then threaded into a vertebra, and once properly positioned, a fixation rod is seated through the rod-receiving portion of each screw and the rod is locked in place by tightening a cap or similar type of closure mechanism to securely interconnect each screw and the fixation rod. Other anchoring devices include hooks and other types of bone screws
Set screws are typically set into location using self-retaining screw inserters or self retaining drivers. These generally use small springs to retain the set screw on the end of the inserter or driver. These springs can deform or break when the inserter or driver experiences too much torque during insertion. Even though the inserter is just for initial insertion of a set screw, frequently surgeons over-torque the inserter causing premature failures. Another type of inserter uses a morse taper. These also are subject to wear and failure when over-torqued. Thus what is needed is a self-retaining screw inserter that does not rely on springs or morse tapers.